1. Field of the Invention
The present invention relates to a portable terminal for measuring a reference tilt and a method of measuring the reference tilt. Particularly, the present invention relates to a portable terminal including a movement sensor for measuring a reference tilt using a gravitational acceleration value and a method of measuring the reference tilt.
2. Description of the Related Art
In recent years, the use of portable terminals such as a mobile communication terminal or personal digital assistant (PDA) has dramatically increased. These portable terminals find wide use in many applications such as making a phone call, managing a schedule, taking a picture with the use of a built-in digital camera, viewing a satellite broadcasting or enjoying a game. Specifically, as the number of games for the portable terminals has significantly increased, these terminals have adapted various shapes and functions suitable for enjoying the games. Generally, a portable terminal is manipulated using a keypad or a touch screen. It can also be manipulated by its' movement and tilt with the inclusion of a movement-detecting sensor. Therefore, when reading a book displayed on the portable terminal, a scroll function can be manipulated by tilting the portable terminal, without manipulating a scroll bar. In addition, new games using this technique are being developed.
A portable terminal for playing a game using its detected tilt as well as a button of the keypad requires a movement sensor to detect the tilt and the movement information. There are various types of movement sensors, and a representative type is a sensor using an accelerometer.
FIG. 1 illustrates an acceleration value measured by a movement sensor. In FIG. 1, the acceleration measured by the movement sensor is a gravitational acceleration. The movement sensor calculates an acceleration Ax(t) of an x-axis 100, an acceleration Ay(t) of a y-axis 110, and an acceleration Az(t) of a z-axis with respect to time (t), and measures the variation of the acceleration according to time. In this manner, the movement sensor measures the tilt and the position change of the portable terminal.
The movement sensor is used as a part of an inertial navigation system (INS) in such vehicles as an airplane, vessel or missile. In the inertial navigation system, a roll angle Roll(Φ) of the x-axis and a pitch angle Pitch(θ) of the y-axis are calculated from Equation (1) below.φ=a tan 2(−Ay,−Az)θ=a tan 2(Ax,√{square root over (Ay2+Az2)})  (1)where a tan 2(a, b) is arg(b+ia), i is an imaginary unit, and arg is an argument of a complex number. Here, Equation (1) above may be modified.
When the portable terminal with the movement sensor is used to enjoy a game, a relative tilt value with respect to a certain state is needed. That is, a starting state in which the user can conveniently manipulate and tilt the portable terminal to conveniently enjoy the game, is required. This tilt will be referred to as a reference tilt.
At a time when a button such as a game start button of the portable terminal is pressed, tilt information of the portable terminal is measured as the reference tilt.
However, when the reference tilt is measured at the press of the button, an accurate measurement of the reference tilt is difficult due to vibration occurring when the button is pressed. This problem becomes more serious in a small-sized terminal because the error increases. Also, if the user is moving when pressing the button, an acceleration component due to the movement is reflected in Equation (1) above and thus the reference tilt cannot be measured accurately.